It was apparent to us that the conductive polymer made of polymer and conductive filler material dispersed in the polymer, and the technique of manufacturing the conductive polymer into the over-current protection device with PTC behavior. Generally, PTC conductive polymer is made of a kind of or more kinds of crystalline polymer and a kind of conductive filler material and the conductive filler material dispersed in the polymer. The polymer can be a kind of or the mixture of more kinds of polyethylene vinyl copolymer and fluoropolymer: The conductive filler material can be black carbon metal particles or inorganic ceramic powder. The PTC behavior of the conductive polymer the resistance value increase with the increasing of temperature) is considered to be due to the rupture of the conducting path, which is made of conducting particle because of the expansion of the liquating crystalline polymer.
Among the existing published technology, the general way is to use the black carbon as the conductive filler material. But the conductive polymer using the black carbon as the conductive filler material is too difficult to get a low room-temperature resistivity, particularly using the polymer to make the over-current protection device of batteries can't satisfy the requirements of the miniaturization (e.g. size 1210, and the device area is 0.12″×0.10″, changing to the metric unit is 3.4 mm×2.75 mm) and the low resistance at room temperature (The typical resistance value of the zero power is 5 mΩ, and the resistance value is less than 15 mΩ after welded.). Nevertheless using the metal particles (e.g. nickel powder) as the conductive filler material can get conductive polymer of lower resistivity at room temperature. The over-current protection device made of the conductive polymer can satisfy the requirements of the miniaturization and the low room-temperature resistance. But there is a new problem: general metal powder is very easy to be oxidized, particularly the oxidizing reaction will speed up in hot environment, which is the cause of the increasing resistance of the device, and it will lead the device to the failure.
To this end, the present invention will publish a surface-mount type overcurrent protection element with subsize low resistance and good environmental stability.